history_container.py 文件源码

def prior_values_index(self):
        index_values = list(
            product(
                (freq.freq_str for freq in self.unique_frequencies),
                # Only store prior values for forward-fillable fields.
                self.ffillable_fields,
            )
        )
        if index_values:
            return pd.MultiIndex.from_tuples(index_values)
        else:
            # MultiIndex doesn't gracefully support empty input, so we return
            # an empty regular Index if we have values.
            return pd.Index(index_values)